A 40-year-old man receiving high-dose intravenous penicillin for endocarditis develops anemia with Hb falling from 13 to 8 g/dL, spherocytes on smear, and a positive direct antiglobulin test showing IgG. Elution studies show antibody bound only to drug-coated red cells. What is the mechanism?
- A Drug acts as a hapten coating red cells, with IgG binding the drug-cell complex ✓
- B Drug induces a true anti-RBC autoantibody reactive even without the drug
- C IgM antibodies against the drug fix complement onto uncoated red cells
- D Immune complexes deposit nonspecifically on red cell surfaces
Correct answer: A. Drug acts as a hapten coating red cells, with IgG binding the drug-cell complex
Explanation
High-dose penicillin causes hapten-mediated immune hemolysis: the drug coats red cell membranes and IgG antibodies bind the membrane-drug complex, producing spherocytes and extravascular splenic clearance. The elution finding of reactivity only with drug-coated cells distinguishes this from methyldopa-type autoantibody hemolysis, where the antibody reacts with Rh antigens even in the absence of the drug. Complement-fixing IgM describes the quinidine immune-complex pattern, not this one.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.